This invention relates to ring laser gyroscopes and in particular to an apparatus and method for compensating a gyroscope output signal to correct for error sources such as optical power variations which produce variations of a polarization split or a dihedral frequency.
Multi-oscillator ring laser gyroscopes are a significant new class of rotation sensing instruments employing four waves of two polarization pairs, each polarization pair propagating in opposite circular directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657, 3,854,819 and 4,006,989 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beams, propagating in the clockwise direction includes both left-hand circularly polarized (LCP) waves and right-hand circularly polarized (RCP) waves as do those waves propagating in the counterclockwise direction. The separation between the LCP waves and the RCP waves in said referenced patents is provided by a crystal rotator which essentially provides a frequency bias (f.sub.B). Such a biased four-frequency or multi-oscillator ring laser gyroscope provides a means for circumventing the frequency locking or lock-in problem present in all conventional or two-frequency laser gyroscopes. This lock-in phenomenon occurs when two traveling waves propagating in opposite directions in a resonant cavity at slightly different frequencies are pulled toward each other to combine in a single frequency standing wave. However, when the frequencies of the counter-rotating waves are sufficiently separated in frequency, the pulling together does not occur. The four-frequency approach may be described as two independent laser gyroscopes operating in a single stable resonator cavity, sharing a common optical path, but static biased in opposite senses by the same passive bias element. In the differential output of these two gyroscopes, the bias then cancels, while any rotation generated signals add, thereby avoiding the usual problems due to drifts in the bias and giving a sensitivity twice that of a single two-frequency gyroscope. Because the bias need not be dithered, the gyroscope never passes through lock-in. Hence, there are no dither-induced errors to limit instrument performance. For this reason, the four frequency gyroscope is intrinsically a low noise instrument, and it is well suited for applications requiring rapid position update or high resolution.
The four different frequencies are normally generated by using two different optical effects. First, a crystal polarization rotator has been used to provide a direction-independent polarization causing the resonant waves to be circularly polarized in two directions. The polarization rotation results from the refractive index of the rotation medium being slightly different for RCP and LCP waves. However, a non-planar ring path is used with this invention which inherently supports only circularly polarized waves without the use of a crystal rotator. The non-planar ring path is sometime-s considered to be a dihedral configuration providing the frequency bias (f.sub.B) or polarization split frequency difference separating the circularly polarized waves; this frequency is also referred to as a dihedral frequency (.DELTA.f.sub.D). A planar electromagnetic wave ring resonator is shown and described in U.S. Pat. No. 4,110,045 to Irl W. Smith, Jr. and Terry A. Dorschner and assigned to the present assignee. Second, a Faraday rotator is used to provide non-reciprocal polarization rotation, by having a slightly different refractive index for clockwise (cw) traveling waves than for counterclockwise (ccw) traveling waves. This causes the cw and ccw RCP waves to oscillate at slightly different frequencies while the cw and ccw LCP waves are similarly but oppositely split. Thus, a multi-oscillator laser gyroscope operates with right circular polarized waves biased in one direction of rotation and with left circular polarized waves biased in the opposite direction, the bias being cancelled by subtracting the two outputs.
An output signal of a ring laser gyroscope drifts with time due to changes in parameters such as temperature and aging. Direct measurement of these parameters generally is not accurate enough or possible. However, gyroscope output accuracy has been improved by measuring the Faraday frequency to sense temperature caused variations and then applying a correction factor to the gyroscope output signal. In this invention, the measurement of the polarization split or the dihedral frequency of a four-frequency laser gyroscope is used to correct the gyroscope output signal for optical power variations and other error sources, such as loss variations due to aging producing a variation of the polarization split or dihedral frequency.